1. Field of the Invention
The present invention relates generally to high pressure ratio turbochargers. More particularly, a turbocharger is provided having a two stage compressor with back-to-back radial flow compressor wheels with integral air flow ducting allowing a reduction in the rotational speed of the turbocharger while increasing pressure ratio over conventional units.
2. Description of the Related Art
Developments in the turbocharger field continue to require increased pressure ratios as a means for providing improved fuel economy, higher power ratings and improved emissions performance for engines on which turbochargers are employed, particularly for commercial diesel application. With conventional turbocharger designs, the sole method for achieving such increased pressure ratios has been to increase the rotational speed of the compressor and turbine components. Current pressure ratio capability for turbochargers of conventional design is typically in the 3.5 range, although some specialized applications operate at 4.0. Currently, the only known method for increasing the pressure ratio capability of a compressor for a given maximum rotational tip speed, is to reduce the backward curvature of the blades. Backward curvature is used to improve the flow range capability of a compressor as well as to improve the efficiency, thus reducing the backward curvature results in less efficiency and a narrower flow range. Requirements for commercial diesel engines for trucking and industrial applications are rapidly approaching pressure ratios of 5 to 6 and possibly higher with flow ranges of over 2.5/1 choke flow to surge flow. Material property limits are exceeded in the rotating components of conventional turbocharger designs at these pressure ratios due to the stresses imposed by the required high rotational speeds. For a turbo using a traditional single stage compressor design, the optimum turbine design for efficiency cannot be used due to the high inertia of a low specific speed design. High inertia reduces the response of the turbocharger to meet the transient requirements of the engine.
Multiple stage compression through the use of two or more turbochargers operating with their compressors in series has been an approach to meeting elevated pressure ratio requirements. However, the cost and complexity of such systems as well as the packaging size requirements are unattractive for most applications. The use of multiple compressor wheels on a common shaft to obtain compression stages, including combining axial and radial compression stages, has also been employed in the prior art. Packaging constraints, particularly length of the turbocharger, rotor dynamics and bearing issues have similarly rendered these designs commercially unacceptable in most cases.
It is, therefore, desirable to provide a turbocharger having a high compression ratio with minimal packaging size increase and reduced complexity over prior art designs. Maintaining a part count comparable to existing commercial diesel turbocharger designs is also desired to keep costs at an acceptable level.
It is also desirable to provide a turbocharger having increased turbine efficiency by reducing turbine rotational speed thus allowing use of a highly efficient low specific speed turbine.